Fatigue crack growth behavior of equiaxed, duplex and lamellar microstructure gamma-base titanium aluminides

Fatigue crack growth behavior of binary and chromium/niobium alloyed gamma-base titanium aluminides with lamellar, duplex and equiaxed microstructures were investigated. Materials with lamellar microstructure exhibited superior fatigue crack growth properties compared with duplex microstructure material, which in turn were superior to equiaxed microstructure material. Addition of chromium or niobium, which refined the grain size, had no significant influence on the fatigue crack growth behavior. Fatigue crack path investigations carried out indicated the contribution of roughness-induced crack closure due to fracture surface mismatch and asperity contacts. Crack-tip shielding due to bridging ligaments was also observed. Both crack closure and crack-tip shielding were significant in the material with higher content of lamellar colonies and enhanced the fatigue crack growth resistance. The basic mechanism of fatigue crack growth seemed to be that microcracks nucleate near the crack-tip and coalesce. Copyright (C) 1996 Elsevier Science Limited